Vectorizing a Loop Using the _Simd Keyword

Vectorization is a key component of Intel Cilk Plus technology. SIMD Vectorization is discussed in some depth in the User-mandated or SIMD Vectorization section of the Auto-vectorization key feature chapter and the simd pragma is documented in the Pragmas section of the Compiler reference Chapter.

In this section we introduce the _Simd keyword which provides an alternative to the simd pragma. Just like the simd pragma, the _Simd keyword modifies a serial for loop for vectorization. The syntax is as follows:

_Simd [_Safelen(constant-expression)][_Reduction (reduction-identifier : list)] 

The _Simd keyword and any clauses should come after the for keyword as in this example:

for _Simd (int i=0; i<10; i++){
  // loop body
}

The _Simd keyword can also be used with cilk_for. See the cilk_for keyword documentation for a discussion of the cases where they are most effectively used together.

Differences between the simd pragma and _Simd keyword:

• Omission of the private and lastprivate clauses of the simd pragma construct because C and C++ already have variable-scoping rules that allow a programmer to cleanly declare a private variable within the scope of a loop iteration
• The linear clause is omitted because the ability to increment multiple variables makes it unnecessary. See the following example:

float add_floats(float *a, float *b, int n){
  int i=0;
  int j=0;
  float sum=0;

  for _Simd _Reduction(+:sum) (i=0; i<n; i++, j+=2){
    a[i] = a[i] + b[j];
    sum += a[i];
  } 
  return sum;
}

To ensure that your loop is vectorized keep the following in mind:

• The countable loop for the _Simd keyword has to conform to the for-loop style of an OpenMP* canonical loop form except that multiple variables may be incremented in the incr-expr (See http://www.openmp.org/mp-documents/OpenMP4.0.0.pdf ).
• The loop control variable must be a signed integer type.
• The vector values should be signed 8-, 16-, 32-, or 64-bit integers, single or double-precision floating point numbers, or single or double-precision complex numbers.
• You cannot use any control constructs to jump into or out of a SIMD loop. That includes the break, return, goto, and throw constructs.
• A SIMD loop may contain another loop (for, while, do-while) in it, but goto out of such inner loops is not supported. You may use break and continue with the inner loop.
• A SIMD loop performs memory references unconditionally. Therefore, all address computations must result in valid memory addresses, even though such locations may not be accessed if the loop is executed sequentially

See Also